A paste vinyl chloride resin is prepared by adding a vinyl chloride monomer alone or a mixture of the vinyl chloride monomer and a monomer which is co-polymerizable therewith (these are broadly referred to as “vinyl chloride monomer”), an emulsifier and a fat-soluble polymerization initiator to an aqueous medium, followed by homogenization and micro-suspension polymerization. If necessary, an auxiliary dispersant such as higher alcohol or higher fatty acid may be used.
Alternatively, the paste vinyl chloride resin is prepared by adding a vinyl chloride monomer, an emulsifier and a water-soluble polymerization initiator to an aqueous medium, followed by performing emulsion polymerization or seed emulsion polymerization.
In accordance with seed emulsion polymerization, when two types of seeds having different mean particle diameters are added in an early polymerization stage, a vinyl chloride monomer is grown while reacting with the seeds to obtain final latex particles.
80 to 98% by weight of the vinyl chloride monomer used for polymerization of a paste vinyl chloride resin is generally converted into the paste vinyl chloride resin and the remaining un-reacted monomer is removed, to obtain a latex. The paste vinyl chloride resin is obtained by spray drying the latex in a spray drier. Steam of a high temperature of 200° C. is required to evaporate 50% by weight or more of water in the latex during drying. An increase in solid content is considerably effective in energy consumption, because a great amount of expensive steam is required to evaporate a great amount of water having a high specific heat. In conventional methods, latex concentration has been used to increase a solid content of latex. Solid content of latex is increased by separating water using a separation membrane. However, this concentration entails high initial investment cost, is difficult to increase a solid content to 3% by weight or higher, has high maintenance costs due to costs required for frequent replacement and cleaning of separation membranes and is considerably ineffective.